He works ok in UU at a rain-rest set, but that's about all he can do:
Grass Knot/Ice Beam
Hydration heals status in the rain, so use it for free recovery and pray they aren't packing golduck. Surf for STAB, Grass Knot for other water types or ice beam for grass types and altaria. If your confident of your team's weather support, you could try scrapping RD and run both the options for the bottom slot.